Evaluating the geoengineering treatment

Might there be a remedy for the worldwide temperature and rainfall changes caused by humanity’s emissions? If so, what would the cure cost? We watch as Mr. Human grapples with these questions with the help of Dr. Planet.

Dr. Planet was about to put an end to a long, hard day of work when the distress call came in.

“Dr. Planet! Dr. Planet! Our planet Earth needs your help!”

Dr. Planet quickly boarded his medical spaceship and sped towards the solar system. As the ship passed through Earth’s atmosphere, his instruments began to gather the planet’s climate records. The temperature indicator began to blink red. Then the indicator for circulation changes in its atmosphere and oceans. Then the sea ice indicator.

“We know that,” said Mr. Human. He sounded as if he had not slept for days. “We’ve known about it for decades. I was so excited after the Paris meeting, when the world first agreed on concrete pledges to cut down emissions. Then we did our sums and realized that even if every country fulfilled its promised reductions, global mean temperatures were still set to increase by more than 2 degrees Celsius come 2100. And then the United States announced that they would pull out of the agreement, which was…”

Mr. Human’s gaze fell as he trailed off. He then straightened and looked Dr. Planet in the eye. “Dr. Planet, you are a renowned planetary climate surgeon. Do you have a geoengineering treatment that might be able to cure our Earth?”

Mr. Human took out a few geoengineering brochures and laid them on Dr. Planet’s desk. They had been produced by the hospital’s marketing department.

Dr. Planet resolved to have a chat with the marketing department about a more moderate portrayal. He was getting tired of patients either believing that geoengineering was a panacea or cursing him for attempting to play God. In fact, the carbon dioxide removal and solar geoengineering tools he possessed only allowed for a limited range of outcomes. More importantly, all of the choices involved tradeoffs and risks. However, experience had taught him that it was best to begin by explaining the science.

Dr. Planet picked up the first brochure. It was about Canadian entrepreneur Russ George, who in 2012 dumped a hundred tons of iron into the ocean to trigger a massive plankton bloom. There were record hauls of salmon right after the fertilization. George also pointed out that the plankton removed carbon dioxide from the air as they grew.

“It’s easy to remove carbon dioxide from the atmosphere,” began Dr. Planet. “The problem is keeping the carbon dioxide out. If the fish is harvested and used as food, the carbon makes its way back into the air. Also, when the plankton respire, or are eaten by organisms higher up the food chain, most of that carbon is released once again. In addition, the immediate phytoplankton growth triggered by fertilization robs the iron or phosphorous that might have been used by other organisms. If you are looking for a long-term solution, don’t get tricked into looking only at the initial gains.”

“Besides, iron fertilization can’t be the only solution. In the most optimistic scenarios, the bulk of the carbon uptake would be used to form the shells of marine organisms such as diatoms. Since the shells would eventually fall to the bottom of the ocean, there would be a net removal of carbon from the surface. But based on the availability of iron-deficient waters around your planet, I estimate that iron fertilization can sequester at most 10% of human annual emissions.”

Mr. Human brightened as he saw that this technology could store about 1,600 billion tonnes of carbon dioxide. If humanity continued emitting at 2014 levels, this would lock up about 45 years of carbon dioxide emissions. When he came to the section on costs, his jaw dropped. “Double the cost of our existing power plants?” He took out his bulging wallet and removed a stack of bills. Dr. Planet wondered if Mr. Human considered this so cheap that he was willing to pay upfront.

Mr. Human waved the bills. “Look at all the IOUs! There is no way we can afford that cost. I’ll bet the aerosol plan is cheaper than that.”

Solar radiation management

Mr. Human pointed to a printout explaining how particles called aerosols could be placed high in the atmosphere. Choosing aerosols that reflected solar radiation would help cool the Earth’s surface.

Dr. Planet understood why Mr. Human liked the aerosol plan. It made sense to place the aerosols far above the surface. That way, it would take many months before the aerosols settled below the clouds, where rain could flush the particles from the air. Furthermore, after the eruption of Mount Pinatubo in 1991, global-mean temperatures in the Northern hemisphere fell by half a degree Celsius. With such a natural analog in mind, it was no wonder that Mr. Human thought he knew what to expect. He even was correct on the costs. Starting from 2040, dedicating 6700 flights a day to sulfate injection would keep global-mean warming to 2 degrees Celsius. This would involve a mass of sulfates roughly similar to that of the Pinatubo eruption and would cost about $US20 billion per year.

Volcanic ash after the eruption of Mount Pinatubo in 1991 (Source: USGS )

“Of course not,” said Mr. Human. “Rainfall is important too. Also, I want to make sure we keep the West Antarctic Ice Sheet, and reduce…”

“Then I should let you know that using aerosols means making a choice between overcorrecting for temperature or precipitation,” said Dr. Planet. He used the same serious tone a human doctor might use to explain that chemotherapy might remove the tumor, but would also cause you to vomit and lose all your hair.

Mr. Human folded his arms. He looked most unconvinced.

As Dr. Planet cast about for a good explanation, his eyes fell on Mr. Human’s wallet. It was still on the table and still full of the IOUs. He picked up a stack of name cards from his table.

“What if I asked you to place all of the cards into your wallet?”

Mr. Human frowned at the thick wad of paper. “I would have to remove some of my old receipts, or the wallet wouldn’t close.”

“Think of the Earth’s surface as the full wallet,” Dr. Planet said. “If we put in energy from increasing sunlight, your Earth has to throw out some energy. Because we’re trying to keep the temperature unchanged, the surface can’t radiate more longwave radiation by warming. It therefore has to transport heat, which mostly happens through evaporation. In the atmosphere, what comes up must come back down eventually, so increasing evaporation increases rainfall.”

“They do,” said Dr. Planet. “But the atmosphere is mostly transparent to solar radiation and mostly opaque to longwave radiation from carbon dioxide. Energy entering via solar radiation thus has a stronger impact on the surface and rainfall. Hence, trying to correct for the change in temperature from carbon dioxide by stratospheric aerosols is expected to lead to an overcorrection in precipitation .”

Mr. Human was silent for a while, before he perked up. “Well, a slight change in the weather we’re used to isn’t that bad, especially if it avoids a worse outcome. Besides, you’ve only talked about the global-mean. With some fine-tuning, I’m sure we could come up with an aerosol distribution that delivers a good balance.”

“However, given the lack of rigorous clinical trials on planets like your Earth, I must warn you that it will remain a highly uncertain procedure,” said Dr. Planet. “For one, we will encounter diminishing marginal returns as we attempt to increase the sulfate load to achieve cooling. The increased amount of sulfate in the atmosphere could form bigger particles that reflect sunlight less efficiently rather than create new ones.”

“The treatment procedure of sustaining the thousands of aerosol-injection flights will require the commitment and coordination of all the peoples of your planet. A disruption due to conflicts could be catastrophic. If the aerosol concentrations are not maintained, the decades’ worth of change from greenhouse gases that they are holding back would manifest in a couple of years. The change would be so sudden that there would be little time for you to adapt.”

Mr. Human paled. Countries might well balk at paying the geoengineering bill. After all, that was money that could go to feeding the poor or to reducing a budget deficit. A rogue country might threaten to disrupt the injections unless sanctions were lifted. Or a country that might benefit from warming could sabotage the flights…

“I think you already know what I’m about to say,” said Dr. Planet as Mr. Human buried his face in his hands. “There’s no magic pill here. There never has been. I can help perform some stopgap surgery by removing carbon dioxide or provide some symptomatic relief through solar radiation management. Ultimately, though, your species has to stop lighting up in the way it has.”

Mr. Human sighed; he had to deliver the sobering news that geoengineering was riskier and more complicated than his colleagues they had expected. As he rose from his chair, he realized that he was still holding his smoking carbon cigarette. The numbers on Dr. Planet’s carbon dioxide detector were still rising. He watched the readout as it went past 400ppm, then 410ppm. With a regretful sigh, he ground the lit end of his cigar into an ashtray and stepped out to continue the long journey ahead.

Acknowledgments: This article was inspired by a group discussion with Dr. Simone Tilmes at the 2017 Princeton Atmospheric and Oceanic Sciences Workshop on Climate Engineering. Katja Luxem and Ben Zhang read an early draft and helped improve the clarity of the article.

Xin is a PhD candidate in Princeton’s Program in Atmospheric and Oceanic Sciences, a collaboration between the Department of Geosciences and the NOAA Geophysical Fluid Dynamics Laboratory. She combines high-resolution models and theory to better understand the changes in tropical rainfall extremes as the atmosphere warms. She is also interested in innovative approaches to science communication.